1. Field of the Invention
The present invention relates to an improved process for catalytic conversion of hydrogen polysulfides to hydrogen sulfide in liquid sulfur and degasification of the liquid sulfur. More specifically, it is concerned with the removal of hydrogen polysulfides and hydrogen sulfide present in liquid sulfur produced by the Claus process. A novel apparatus suitable for carrying out this process is also shown.
2. Description of the Prior Art
The toxicity and combustion hazards associated with gaseous hydrogen sulfide are well recognized and documented in the literature. Aside from being a noxious gas in relatively dilute concentrations, hydrogen sulfide becomes progessively dangerous as its concentration in a gas exceeds about 70 wppm, the fatal concentration for man being about 700 wppm. When the hydrogen sulfide content of a gas increases to at least 3.4 volume precent (which corresponds to an equilibrium value of 15 wppm hydrogen sulfide in liquid sulfur) in the presence of an oxygen containing gas such as air, the inflammability threshold of the gas will be exceeded. In addition, should liquid sulfur be confined in a vessel having iron as a material of construction, the iron sulfide formed on the inner walls thereof may be pyrophoric and in the presence of an oxygen containing gas, may result in spontaneous ignition of hydrogen sulfide without any flame or spark being present. Thus, it is desirable to maintain the concentration of hydrogen sulfide in liquid sulfur below 15 wppm.
Dissolved hydrogen sulfide is not only an absorption process. Hydrogen sulfide is known to combine with sulfur to form hydrogen polysulfides and such polysulfide formation is favored in the high temperatures associated with the Claus plant. The conversion of polysulfides back to hydrogen sulfide is an extremely slow process. Thus, the polysulfides are inherently produced in the Claus process, and once formed are extremely slow in decomposing.
Canadian Pat. No. 1088276 discloses a process for the removal of hydrogen sulfide and hydrogen polysulfide from liquid sulfur using a solid catalyst particularly suitable for the conversion of the polysulfide to hydrogen sulfide. In this process, the liquid sulfur containing the contaminants is treated with a solid degradation catalyst selected from the group consisting of alumina and cobalt-molybdenum impregnated alumina catalyst at a temperature from about 250.degree. F. to about 320.degree. F. while purging said liquid sulfur with an oxygen containing purge gas. The resulting hydrogen sulfide laden purge gas is separated from the liquid sulfur.
A number of multicompartment degassing processes have been disclosed in the art. Two of these, the Shell and SNEA degassing processes, are described in Sulphur No. 173, July-August 1984, pages 36-37, published by The British Sulphur Corporation Limited.
In the Shell system, degassing is carried out in a degassing compartment in the Claus plant collecting pit. Degassing occurs in a stripping column located in this collecting pit. While this sweep gas does remove hydrogen sulfide from the sulfur, long treating times are necessary in view of the slow conversion of hydrogen polysulfide to hydrogen sulfide.
The SNEA degassing system using two compartments with continuous circulation and spraying of the sulfur into the collecting pit through which is passed a sweep gas for hydrogen sulfide removal. In both the Shell and SNEA processes, ammonia, a well known catalyst, can be used.
A multistage degasification process for removing hydrogen sulfide from liquid sulfur is shown in Campbell, et al., U.S. Pat. No. 4,131,437 (1978) wherein the sulfur passes through a series of stages and in at least one stage, the sulfur is sprayed from a lower portion in the chamber into the purged gas space above. Air is passed through this zone for hydrogen sulfide removal. In this respect, the system is somewhat related to the SNEA process described above.
An object of this invention is to provide an improved process for the conversion of hydrogen polysulfide to hydrogen sulfide in liquid sulfur and removal of hydrogen sulfide from said liquid sulfur.
Another object is to provide apparatus for carrying out this process.
Other objects and advantages of the invention will be apparent to one skilled in the art upon reading this disclosure.